Yes, 28 grains would be a 3 1/2 Dram Eq. load of Ballistite
http://books.google.com/books?id=97dJAAAAYAAJ&pg=PA57&lpg#v=onepage&q&f=false ..
BULK.....................................................
DENSE.............'E.C.'......DuPont...Schultze...Walsrode Green......Ballistite.Infallible
3 Dram....42...........36.5..............42................30.....24..............21 grains
3 1/4....45.5.........41................45................32.....26..............23 grains
3 1/2....49...........43................48................34.....28..............25 grains
More pressure information here
https://docs.google.com/a/damascusknowle...fUOZEFU/preview 
